1. Field of the Invention
The present invention relates to a solid electrolyte for use in a gas sensor element in a gas sensor, which is capable of detecting a concentration of a specific gas component contained in an exhaust gas emitted from an internal combustion engine such as diesel engines. The present invention further relates to a method of producing the solid electrolyte, and to a gas sensor equipped with a gas sensor element using the solid electrolyte.
2. Description of the Related Art
A gas sensor equipped with an exhaust gas sensor element such as a O2 gas sensor element, a NOx sensor element, and an A/F (air/fuel) sensor element is used to detect a concentration of O2 gas and a concentration of NOx gas contained in, and an A/F ratio of an exhaust gas emitted from an internal combustion engine such as diesel engines and gasoline engines mounted on vehicles. Each of those exhaust gas sensor elements use a solid electrolyte made of zirconia (or zirconium), for example.
In general, various types of stress are applied to the gas sensor element placed in an exhaust gas passage in an exhaust gas system connected to an internal combustion engine. For example, rapid activation of the gas sensor element causes a rapid temperature rise of the gas sensor element, and a thermal stress is given to the gas sensor element. Contacting with moisture or drop of water contained in an exhaust gas or ambient air generates stress in the inside of the gas sensor element. Still further, a rapid temperature change of the exhaust gas or a rapid change of the exhaust gas flow also generates stress in the inside of the gas sensor element.
When stress such an excess stress of not less than a predetermined allowable value is applied to the gas sensor element, a solid electrolyte in the gas sensor element breaks. The malfunction of the solid electrolyte in the gas sensor element cannot correctly detect a concentration of O2 gas and a concentration of NOx gas contained in, and an A/F ratio of the exhaust gas. This decreases the reliability of the gas sensor element.
When the gas sensor element is placed under a condition at a low temperature within a range of 200 to 300° C., zirconia (or zirconium) forming the solid electrolyte transforms in phase from T phase (tetragonal phase) to M phase (monoclinic phase). In that phase transition, zirconia slightly expands in volume by approximately 4%. Expanding the volume of the gas sensor element often generates cracks in the inside of the gas sensor element. Thus, the conventional gas sensor element has such a low-temperature problem.
In addition, there is a possibility for an A/F sensor to receive a thermal shock generated by the presence of water drops contained in the exhaust gas immediately after the engine starts. However, a conventional A/F sensors do not have an adequate thermal shock resistance. In order to avoid the conventional problems of the gas sensor element described above, conventional techniques, for example, Japanese patent laid open publication No. JP H08-15213 delays the activation of an A/F sensor in order to avoid thermal shock.
Further, Japanese patent publication No. JP 2703207 discloses a technique to produce zirconia composite sintered ceramics having high mechanical strength and fracture toughness by using nano-composite material. Such nano-composite material is obtained by diffusing nano-alumina into zirconia and partially stabilized zirconia (which uses ceria and titania (or titanium dioxide) as stabilizing agent).
Still further, the technique disclosed in Japanese patent laid open publication No. JP H11-310456 shows solid electrolyte composite sintered ceramics having high mechanical strength and fracture toughness, and high ion conductivity by using nano-composite material capable of dispersing ceramic grains such as SiC, AIN, BN, ZrB2, and Si3N4 into stabilized zirconia grains (which use yttria as stabilizing agent).
On the other hand, in order to avoid thermal shock, conventional techniques such as Japanese patent laid open publication No. JP H08-15213 disclose delaying the activation of the A/F sensor. However, this conventional technique decreases the efficiency of purifying the exhaust gas during the period of avoiding thermal shock because it is difficult to perform the A/F control during the period of avoiding thermal shock.
Further, because another conventional technique disclosed in Japanese patent publication No. JP 2703207 disperses alumina grains into zirconia grains, this increases electric resistance of the zirconia grains. Therefore it is difficult for that technique to adequately obtain the ion conductivity of the zirconia composite sintered ceramics.
Still further, like the technique disclosed in the Japanese patent publication No. JP 2703207 described above, because the technique disclosed in the Japanese patent laid open publication No. JP H11-310456 disperses alumina grains in zirconia grains, this technique also increases the electric resistance of the zirconia grains, and cannot adequately keep the ion conductivity of the solid electrolyte composite sintered ceramics. Further, because those grains are sintered or fired in non-oxidative atmosphere, oxygen contained in alumina grains in contact with a mixture of non-oxide and zirconia grains is lost, and this decreases the mechanical strength of the solid electrolyte composite sintered ceramics. It is therefore difficult for the solid electrolyte composite sintered ceramics to have adequate thermal shock resistance.